This present invention relates to an improvement for gas-fired radiant tubes used to heat industrial furnaces
Radiant tubes for this application are well known. In radiant tube furnaces, the products of combustion from a burner are confined within a tube with the material to be heated neighboring or surrounding the tube. The heat from the burner is thus indirectly transferred to the material via the tube, primarily by radiation and secondarily by convection. This system isolates the material being heated from the products of combustion of the burner, allowing a protective atmosphere to be maintained surrounding the material.
The radiant tubes can be fired from one end or fired alternately from either end. One end firing single burner radiant tubes are simple--needing only a source of gas and air in order to operate. However, this simplicity comes at a cost. The heat transfer rate for radiant tube heaters is limited by the maximum temperature the tube can withstand without premature tube failure. With a single burner a typical u-shaped or w-shaped tube might have a temperature of 2000F. in the hottest portion and a temperature of 1850F. in the coldest portion. If the load being heated is at 1750F., and for the purpose of this example taking no account of emissivity effects, the black body radiation from the 2000F. portion of the tube to the load is 21,960 Btu per hour per square foot of tube surface while the black body radiation from the 1850F. portion of tube is only 7,950F. Btu per hour per square foot of tube surface. The single burner radiant tube thus has problems with temperature and heat transfer rate uniformity and the accompanying difficulties (shortened tube life, contaminant buildup, etc.). If the coldest portion could be maintained at, or close to, the same 2000F. as the hottest portion, the total heat transfer to the load would be substantially increased without shortening the tube life. Firing a tube with burners at either end can accomplish this by reducing the total temperature spread on the tube to 30 to 40 degrees F. Burners at either end of the radiant tubes therefor improve temperature uniformity and consequently can increase the total heat transfer rate through the tube. They in addition can effect fuel savings by allowing the incorporation of regenerative beds with the burners. (Regenerative heat storage beds absorb and store heat from the products of combustion when the burner is in the flue mode. In the firing mode, the bed transfers some of the stored heat to the combustion air, increasing the overall efficiency of the device). However, the temperature and heat transfer rate uniformity of firing a tube with burners at either end comes at a cost of burner control complexity; in most either end firing radiant tubes the gas, combustion air and the exhaust air all must be selectively valved in order to provide for the customary alternate firing cycle. The construction of these valves, especially the exhaust air valve, is complex. The valves are prone to fail. The time and order of the selective valving also limits the operation of the radiant tube. In addition a source of supplementary combustion air is frequently needed. Therefor the temperature uniformity and other advantages of alternate firing burners at either end of a radiant tube come at a cost in the initial acquisition price, in ongoing maintenance, and in restricted operating parameters.